1. Field of the Invention
The present invention relates to an improved process for polymerizing ethylene or a mixture of ethylene and a small amount of another .alpha.-olefin (hereinafter referred to as simply ethylene) in two successive steps by using a catalyst having high catalytic activity.
More particularly, it relates to a process for producing polyethylene having excellent characteristics for an extrusion molding technique such as hollow molding, in high productivity while controlling the average melecular weight and molecular weight distribution of the product.
2. Description of the Prior Arts
Recently, it has been proposed to prepare a catalyst system for a polymerization of ethylene which has high catalytic activity by combining an organometallic compound with a solid catalytic component obtained by reacting a transition metal halide with various carriers in order to increase the efficiency of the Ziegler type catalyst systems which comprise a transition metal halide and an organometallic compound to gain economic advantages for the polymerization of ethylene.
The inventors have proposed catalyst systems having high catalytic activity which comprises a catalytic component of a reaction product obtained by reacting a specific organic silicon compound with an organic magnesium compound in Japanese Patent Publications Nos. 19308/1975, 19309/1975, 154/1976 and 13232/1977.
In these prior inventions for polymerizing ethylene by using the catalyst systems, catalytic activities for the transition metal component or the solid catalytic component are remarkably high whereby a step of a catalyst-decomposition and separation can be eliminated and the solvent purifying step can be simplified all of which substantially contribute to a reduction in production costs.
However, the molecular weight distribution of polyethylene produced by using such catalyst system is too narrow, so that the product polyethylene is preferably used in special applications such as in injection molding and rotary molding. The polyethylene is not suitable for use in such areas as extrusion molding and hollow molding.
Usually, when a polyethylene having a narrow molecular weight distribution is molded by an extrusion molding or a hollow molding process a, high extrusion velocity can not be expected in the molding operation which results in low productivity and a product of rough texture whose surface is like shark skin or contains, streaks and pits and whose surface is of uneven thickness. These are serious disadvantages in practical applications.
The phenomenon of a rough surface texture of molded products highly related to the molecular weight distribution of the polyethylene. The rough texture of the molded product improves as the molecular weight distribution of the polymer is broadened.
The flow ratio (FR) is used as the practical rating for showing the molecular weight distribution degree of a polymer product. The flow ratio (FR) is shown as FR=MI.sub.10 /MI.sub.2 wherein MI.sub.2 and MI.sub.10 respectively represent melt indexes at 190.degree. C. under a load of 2.16 kg and 10.0 kg in the method of ASTM-D 1238-57T. Among polymers having a similar MI.sub.2 value, a polymer having a higher FR has broader molecular weight distribution.
The molecular weight distribution of a polymer can be slightly broadened according to the kind of organometallic compound used as a cocatalyst or by varing the concentration of a molecular weight regulator in the polymerization process in the cases where a catalyst systems is employed. However, it has not been satisfactory in a practical process.
The inventors have studied to improve a molecular weight distribution of a polymer which is the disadvantage of the prior arts. The present invention has been attained by these studies.